The present invention relates generally to image sensing technology. More specifically, the present invention relates to a CMOS image sensor having a novel layout that uses specially-matched microlenses.
Traditionally, imaging technology has been centered around charge coupled device (CCD) image sensors. However, recently, CMOS imaging technology has become increasingly the technology of choice. There are a variety of reasons for this progression. First, CCD imagers require specialized facilities, which are dedicated exclusively to the fabrication of CCDs. Second, CCD imagers consume a substantial amount of power, since they are essentially capacitive devices, which require external control signals and large clock swings to achieve acceptable charge transfer efficiencies. Third, CCD imagers require several support chips to operate the device, condition the image signal, perform post processing and generate standard video output. This need for additional support circuitry makes CCD systems complex. Finally, CCD systems require numerous power supplies, clock drivers and voltage regulators, which not only further increase the complexity of the design, but also demands the consumption of additional significant amounts of power.
By contrast, CMOS imagers are characterized by a less complex design. The more simple architecture translates into a reduction in engineering and production costs and a concomitant and substantial reduction in power consumption. With today's sub-micron CMOS fabrication processes, CMOS imagers have also become highly integrated. For example, an entire CMOS-based imaging system, such as a digital camera, can be fabricated on a single semiconductor chip. Additionally, unlike CCD imagers, CMOS imagers are amenable to fabrication in standard CMOS fabrication facilities. This adaptability significantly reduces plant overhead costs. For these reasons, CMOS imagers are swiftly becoming the imagers of choice.
An image sensor is comprised of an array of picture elements or “pixels.” A layout for an exemplary CMOS unit pixel 10 is shown in FIG. 1. Unit pixel 10 is comprised of a rectangular image sensing area 100, transfer transistor 102, floating node 104, reset transistor 106, drive transistor 108, select transistor 110 and output 112. Unit pixel 10 is powered by power supply VDD 114. Image sensing area 100 is made rectangular to maximize the “fill factor,” which is defined as the percentage of the unit pixel 10 area occupied by image sensing area 100. A typical fill factor for the arrangement of FIG. 1 is approximately 30%.
Referring now to FIG. 2, there is shown a plan view of a partial array of pixels 20, according to conventional CMOS image sensor devices. By positioning hemiscylindrically-shaped microlenses 203 over image sensing areas 200, the effective fill factor for the layout of FIG. 1 can be improved, as incident light 204 is focused more towards the center of rectangular image sensing areas 200 by microlenses 203. The percentage of each unit pixel 20 occupied by each image sensing area 200 does not, of course, change by employment of microlenses 203. Nevertheless, light capture is improved and the effective fill factor is increased. Use of hemiscylindrically-shaped microlenses 203 can increase the effective fill factor to approximately 75%.
Despite the improvement in effective fill factor using hemicylindrically-shaped microlenses, there are negative performance factors, which are attributable to use of rectangular-shaped image sensing areas and hemicylindrically-shaped microlenses.
First, referring to FIG. 2, whereas utilization of hemicylindrically-shaped microlenses 203 is effective at directing incident light 204 arriving at angles perpendicular to the major axes (major axes are in x-direction) of the lenses 203, the hemicylindrically-shaped microlenses 203 are not very effective at focusing incident light 204 arriving at angles non-perpendicular to, i.e. oblique to, the major axes of the hemicylindrically-shaped microlenses 203. This ineffectiveness further involves light that is scattered and/or reflected and which ultimately arrives at the lenses 203 at oblique angles.
The ineffectiveness of hemicylindrically-shaped microlenses 203 to focus incident light towards the center of the image sensing areas 200 is problematic due to the fact that neighboring rectangular-shaped image sensing areas are in close proximity in the x-direction See FIG. 2, where the horizontal spacing between neighboring pixels is shown to be approximately 0.8 μm. The close proximity results in random diffusion of photocharge generated outside the photodiode depletion regions. Photocharge that is generated outside the depletion region of a particular pixel is prone to capture by a neighboring pixel. When photocharge is unwantedly captured by an adjacent pixel, electrical crosstalk occurs resulting in a reduction in image sharpness.
There is another type of crosstalk that can be attributed to the close proximity of neighboring rectangular-shaped image sensing areas 200. This second type of crosstalk occurs between pixels of different colors and is referred to in the art as “color crosstalk.” Color crosstalk leads to color distortion and is caused by the fact that silicon-based photodiodes have a wavelength-dependent photon absorption response. In particular, color distortion can be significant for image arrays that use rectangular-shaped image sensing areas together with an RGB Bayer pattern color filter. In fact, a difference on the order of 10% in the response of GR (green pixels adjacent red pixels) and GB (green pixels adjacent blue pixels), under uniform illumination, is observed when rectangular-shaped image sensing areas are used. This difference in green pixel responsivity results in color distortion.
Finally, yet another problem that is observed when neighboring image sensing areas are too closely positioned, is a decrease in spatial resolution. In imaging systems, resolution is quantified in terms of a modulation transfer function (MTF). The lower the MTF, the less capable an imaging device is at picking up the fine detail and contrast in the object that is being imaged.
The above problems associated with use of rectangular-shaped image sensing areas 200 and hemicylindrically-shaped microlenses 203 are further discussed in a paper entitled An Improved Digital CMOS Imager, presented at the 1999 IEEE Workshop on Charge-Coupled Devices and Advanced Image Sensors on Jun. 10–12, 1999. This paper is hereby incorporated by reference.
Based on the foregoing deficiencies associated with currently available CMOS imagers, what is needed is a new CMOS imager characterized by low electrical and optical pixel-to-pixel crosstalk, high effective fill factor, and high MTF. The present invention fulfills these needs by providing a CMOS imager having a novel unit pixel layout using specially matched microlenses.